A2L Item 070
- Description: Identify the number of forces acting on a block.
- Goal: Recognizing the presence of forces.
- Source: UMPERG
- Keywords: Forces, Friction, Mechanics, Springs, Tension
The question for students:
A block having mass m moves along an incline having friction as shown in the diagram above. As the block moves a small distance along the incline, how many forces act on the block?
- One force
- Two forces
- Three forces
- Four forces
- Five forces
- Six forces
- Seven forces
- None of the above
- Cannot be determined
Commentary for teachers:
Answer
(5) Five forces act on the block: gravitation, rope, spring, kinetic friction (because you are told the block moves), and normal due to the incline. Many student errors are due to the failure to identify all of the forces acting on a body.
Background
It is helpful to classify forces into action-at-a-distance forces, such as gravity and electromagnetism, and contact forces. Students can then employ a strategy for identifying all the forces since every object touching a body will give rise to a force. The only exceptions are the fundamental forces, which is an easily exhausted list.
Questions to Reveal Student Reasoning
Does it matter if the block is moving up the plane or down? If the block is at rest, how many forces MUST be acting on the block? How many forces may be acting but you can’t be sure?
Suggestions
Set up some situations with blocks, springs and ropes and let students practice identifying all the forces. This is a good activity to do in conjunction with drawing free body diagrams.